This invention relates generally to power supply systems that performs DC-DC conversion operations, and more particularly, this invention relates to a constant voltage reset circuit for the forward converter to operate the converter more efficiently in applications of the holdup time requirement and the wide input voltage range operation.
DC-DC converters are most commonly used in various power supplies. The input of the DC-DC Converter can be a fixed DC voltage with pre-regulator or a wide range DC voltage.
FIG. 1a shows the conventional configuration and FIG. 1b to FIG. 1d show key operation waveforms of the dual switch forward DC-to-DC converter. When the two switches, S1 and S2, turn on simultaneously, the primary winding of the transformer Tr is magnetized by the input voltage Vin. And when the two switches S1 and S2 turn off simultaneously, the transformer winding is demagnetized reversely by the input voltage Vin through two clamping diodes Da1 and Da2. The reset voltage is equal to the input voltage. In order to keep the voltage-second balance of the transformer winding, the duty cycle of the forward converter is limited within 50%.
A drawback associated with foregoing DC-DC converter is that the converter operates in a small duty cycle with low efficiency under normal high input, since the converter is designed at minimum input voltage with a limited maximum duty cycle of 50%. The smaller the duty cycle is, the smaller the turn ratio of transformer will be, and thus the larger the RMS current of the primary side is, the larger the conduction loss and the switching loss of the main switches will be. Meanwhile, the smaller the turn ratio of transformer is, the higher the voltage rating of the second side of the rectifier with the higher conduction voltage drop and the bigger size of output filter will be.
It is an object of the present invention to extend the switch duty cycle to ensure a higher conversion efficiency for wide input operation range.
It is another object of the present invention to minimize the bus capacitor for a given holdup time requirement.
It is another object of the present invention to lower the voltage rating and to lower the conduction loss of the second side of the rectifier.
According to an aspect of the present invention, a constant voltage reset circuit for a forward converter is proposed. The constant voltage reset circuit comprises a reset capacitor, a parallel-coupled switch, and a diode coupled in series with the reset capacitor, which is configured to cooperate therewith to reset energy from the transformer.
According to another aspect of the present invention, a constant voltage reset circuit is employed on the isolated power supply of a transformer of a forward converter. The constant voltage reset circuit is capable of demagnetizing the transformer and is further capable of recovering the energy stored in the transformer with a constant voltage reset to extend the switch duty cycle higher than 50% at runnable minimum input to improve the overall performance of the converter.
In one embodiment of the present invention, the constant voltage reset circuit further comprises a logic control unit coupled to the switch which is configured to enable the reset voltage of the forward converter to be almost constant even when the input of the forward converter drops to a lower voltage.
In one embodiment of the present invention, the diode is selected from a separated diode, a zener diode and a body diode of the switch such as a MOSFET.
In one embodiment of the present invention, a resistor is further connected in parallel with the diode.
In one embodiment of the present invention, the logic control unit includes a zener diode and a transistor biased through the zener diode.